This invention relates to ammonium nitrate bodies produced by spraying a liquid mixture of ammonium nitrate and water that is highly concentrated in ammonium nitrate (herein xe2x80x9cANxe2x80x9d) in to an atmosphere where the AN rapidly crystallises and water is removed by evaporation. The bodies may be prills (i.e. dense round particles resulting from crystallisation of free-falling droplets of mixture) or agglomerates (i.e. less dense associations of the component AN crystallites, irregular in shape and dimensions) or granules (i.e. generally round particles grown by spraying the mixture on to tumbling seed material such as small AN prill so that layers of AN crystals become encrusted on the seed material).
The invention is particularly concerned with ammonium nitrate bodies for use in explosives products, but is not restricted to such bodies. In particular, the invention may also extend to such bodies for use in fertiliser products.
Descriptions of technologies for the production of AN prills and pan-granulated AN bodies are available in the patent and other literature. Reference may usefully be had to EP-A-0320153 (acoustic frequency vibratory prilling to produce a substantially mono-sized prill), U.S. Pat. No. 5,354,520 (intensive prilling process) and U.S. Pat. No. 5,395,559 (pan-granulation to fatten mono-sized small prills). The preferred size for AN prills used in explosives products is about 2-3 mm; the preferred size of granulated products is about 3-4 mm and for these a 1 mm seed prill would be suitable.
AN prill intended for use as oxidiser in explosives products is required to have a sufficient porosity to allow absorption of from 6 to 12% by weight of fuel oil and yet be a particulate free-flowing product which can be poured or augered into bore-holes. This product is so-called ANFO. Such a prill is also a very satisfactory material for blending with emulsion explosive. To achieve such porosity, the AN/water mixture should contain up to, at most, around 97.5% by weight of AN, the preferred range being 92-97.5%, more preferably 94-96% by weight AN. The water content of the AN/water mixture also influences the density and friability of the AN prill. Thus, for instance, a fertiliser grade AN prill which is produced from an AN/water mixture containing at least 98% by weight AN is hard, dense and of low porosity. Other characteristics of an explosives grade AN prill that influence friability and stability towards atmospheric changes (e.g. temperature cycling and humidity) are the micro-crystalline structure and the residual moisture content of the prill. These characteristics are important also in other forms of AN bodies used as solid oxidiser in explosives products, e.g. pan-granulated AN particles, although they would not possess the hardness or low friability desired in an explosive grade AN prill. It is known to incorporate crystal-habit modifiers in AN/water mixtures used as feed in the described processes to encourage or cause the AN to crystallise as small crystals of preferred conformation. As a general rule, smaller crystals of lower aspect ratio are a feature of stronger, less friable AN bodies, especially prills. U.S. Pat. No. 5,597,977 describes the use of poly-styrene sulphonate as an ingredient in AN/water mixtures used to produce AN bodies.
Aside from residual water, minor impurities and minor amounts of deliberately added processing aids and product improvers, the AN bodies intended for use as explosives"" oxidant will preferably consist of AN. It is, however, known to substitute some of the AN by alkali- and/or alkaline metal nitrates in explosives"" oxidisers, and references herein to AN should be construed as AN optionally with a proportion of the ammonium nitrate substituted by one or more such other nitrate oxidant. Thus, references to AN proportions in the feed mixtures may for practical purposes be read as proportions of total nitrate oxidant.
According to the present invention, there is provided a process for the production of ammonium nitrate bodies in particulate form wherein an ammonium nitrate/water liquid mixture containing at least 92% by weight ammonium nitrate, optionally with a proportion of the ammonium nitrate substituted by alkali- and/or alkaline earth metal nitrate, and a small amount of poly-styrene sulphonate crystal-habit modifier is sprayed as droplets in to an atmosphere in which cooling effects crystallisation of the ammonium nitrate and in which the crystallised particles are dried to remove moisture characterised in that the liquid mixture also contains ammonium sulphate, the concentration of poly-styrene sulphonate in the liquid mixture is at least 0.01% by weight, and the concentration of ammonium sulphate in the liquid mixture is greater than that of the poly-styrene sulphonate and is at least 0.04% by weight.
The present invention further provides a process for producing AN bodies by prilling, pan-granulation or other means of production of AN particles composed of aggregates of AN crystals in which an AN/water mixture as immediately hereinbefore described is sprayed with cooling to effect crystallisation of the AN followed by drying in a stream of gas, especially air, to remove substantially all free moisture, preferably to a residual water content of at most 0.1% water by weight in the dried product. The process type is preferably prilling under conditions of acoustic frequency vibratory prilling such that substantially mono-sized prills are produced, with drying effected in one or a train of two or more rotary driers under co- or counter current drying gas, e.g. air, flow, and the process is preferably in accordance with the immediately preceding paragraphs.
The present invention also provides AN bodies, especially AN prills, produced by a process as described in either of the two immediately preceding paragraphs.
The invention also extends to the use of a synergistic combination of poly-styrene sulphonate and ammonium sulphate as crystal-habit modifier in the production of ammonium nitrate prill, especially explosive-grade ammonium nitrate prill.
The AN/water mixture preferably contains at most about 97.5%, more preferably 94-97%, even more preferably 95-97%, by weight of AN. However, the AN/water mixture may contain more AN if the AN bodies are to be used in fertiliser products.
Preferably, the concentration of poly-styrene sulphonate is at least 0.02% by weight of the mixture.
The mixture further includes a quantity of ammonium sulphate which is greater than the quantity of poly-styrene sulphonate present, preferably at a concentration in the mixture of, say, 0.06 to 0.15% by weight, more preferably 0.08 to 0.13% by weight.
The poly-styrene sulphonate may be considered to be present as its ammonium salt, given the preponderance of ammonium ions present in the mixture and the normal practice of pH adjustment to a value of around 5.5 by ammonia addition. Poly-styrene sulphonate exists as a range of molecules, not merely because as a polymer there will be a range of chain lengths determined by the polymerisation process and any fractionation, but also because the degree of sulphonation of polymer chains can vary widely. Thus, at one end of the spectrum of possible molecules is poly (vinyl benzene sulphonate) and descending from that best characterised species are a range of sulphonated poly-styrenes. These should preferably be sulphonated to a level of at least 50% relative to poly (vinyl benzene sulphonate), more preferably at least 75%, and most desirably at least 90%. We have produced sulphonated poly-styrenes with more than 90% degree of sulphonation from a range of poly-styrenes with number average molecular weights from 40,000 to 200,000; all were effective additives. Higher molecular weight materials are more viscous and this can make production and handling more difficult. Low molecular weight material has been produced by styrene polymerisation followed immediately by sulphonation without any intermediate separation or purification stages. These materials had molecular weights centred around 10,000 to 20,000 and again were found to be effective. Indeed, following sulphonation, reaction product may be neutralised with ammonia and the result then is a mixture of the sulphonated polymer and ammonium sulphate which may be used in the process of the invention to produce the ammonium nitrate bodies. Typically, the ratio of polymer to ammonium sulphate by weight may be in the range of from about 1:5 to about 1:10, and this range is also very suitable for mixtures made by mixing separately obtained components.
The ability to use crude mixtures of sulphonated poly-styrene and ammonium sulphate (from liquor neutralisation with ammonia) is significant in economic terms because poly-styrene sulphonates are not readily available items of commerce and to the extent they may be available they will be expected to be specific materials made with quite different end uses in mind.
The observed effect of using poly-styrene sulphonate alone in minor amounts as a crystal-habit modifier (say, at a concentration by weight in the AN/water mixture fed to the spray head of 0.06%) is that it permits production of AN bodies having a lower standing water content than is achievable under similar regimes of thermal/humidity gradient using ammonium sulphate or other known crystal-habit modifiers alone. Thus, whereas a residual water content of, at best, 0.1% by weight would typically be observed for product prill made using other crystal-habit modifiers, the use of poly-styrene sulphonate enables AN prill to be recovered with a water content as low as 0.05%, even 0.03% by weight. We believe the reason for this effect is that the polymeric sulphonate both facilitates free water transport to exposed AN particle surfaces so that moisture removal is faster in the drying stage(s) and acts to xe2x80x9cspringxe2x80x9d that proportion of the contained water which is more tenaciously held by the ammonium nitrate and so free it for transport and evaporation. Surprisingly, when we trialled crude batches of laboratory synthesised sulphonated polystyrene still in admixture with substantial amounts of ammonium sulphate (resulting from ammonia neutralisation of the spent acid) we observed, firstly, no interference with the performance of the sulphonate even though ammonium sulphate is a crystal-habit modifier. Experience teaches the opposite. We then investigated the effect of reducing the proportion of crude mixture used in AN prilling with 94-96% AN liquor. We were astonished to discover that concentrations of sulphonate down to 0.02% by weight of the liquor were acceptable in pilot operations (at 60 kg/hr AN liquor) and even 0.01% by weight was functional at higher commercial scale prilling feed rates (above 10 te/hr). Used alone the sulphonate would not have been effective at such low levels. Nor would the use of ammonium sulphate alone have given the observed results. Clearly there was synergy. The same result was obtained using made-up mixtures of sulphonate and sulphate in similar ratios viz: from 1:5 to 1:10 sulphonate to sulphate by weight. Thus, we have discovered that the advantages which flow from the use of poly-styrene sulphonate alone at an effective level in AN liquor feed in the described processes for the production of AN bodies in terms of moisture removal rate, final water content (when desired), particle integrity and oil absorption can be obtained by using a synergistic combination of poly-styrene sulphonate and ammonium sulphate such that the concentration of sulphonate in the AN feed liquor is from 0.01 to, say, 0.06% by weight and the concentration of ammonium sulphate is from 0.04 to, say, 0.15%. The optional upper values in these stated concentration ranges merely reflect economic preference. At levels of sulphonate of more than 0.06% the advantage of synergy may not be enjoyed, but the advantage of being able to use crude neutralised sulphonate batches containing substantial amounts of ammonium sulphate is still real. Likewise, where the synergistic effect is to be exploited, the use of levels of ammonium sulphate above 0.15% by weight in the AN liquor, though technically acceptable, is wasteful. It is our experience that optimised poly-styrene sulphonation processes do not give rise to relative levels of spent acid (sulphuric acid) such that after ammonia neutralisation there is a gross excess of ammonium sulphate in the product mixture. This is the case whether pre-selected poly-styrene is sulphonated or styrene is first polymerised and then sulphonated in a continuous process without isolation of the poly-styrene.
Reference has been made to the level of residual water in produced AN bodies. This is especially important in our experience when the AN bodies are prills. Ammonium nitrate crystals are thermodynamically stable in different crystal structures within different temperature bands. The transition from so-called Phase IV to Phase III occurs at around 32 degrees Celsius. The presence of moisture appears to encourage that transition so that for the usual AN prill familiar to explosives producers a sequence of hot day/cold night storage frequently leads to prill fracture, increased friability and consequent creation of fines (powdered AN). Product handling, storage, transport and explosives performance are all adversely affected. At low residual water levels in the prill, i.e. below 0.1% by weight there appears to be a significant kinetic impediment to the problematic transition. Initial prill hardness achievable by the use of poly-styrene sulphonate as crystal-habit modifier would not seem alone to be a sufficient reason for stability under repeated thermal cycling to the extent observed.
An incidental and commercially important advantage of lower poly-styrene sulphonate usage in the production of AN bodies is that when the bodies are used to make AN liquor for the production of emulsion explosives there is less chance of emulsion stability being adversely affected by interference of the polymeric sulphonate (a surfactant) with the emulsifier.
Methods of Synthesis of Poly-styrene Sulphonate
Poly-styrene of desired molecular weight is dispersed as a fine powder in 98% strength sulphuric acid containing silver sulphate catalyst at a temperature in the range 98 to 120 degrees Celsius. The powdered poly-styrene is added with intensive stirring to ensure dissolution. Reaction proceeds in the course of this addition and then subsequently for a sufficient period to achieve the desired or maximum degree of sulphonation. Reactor product is neutralised with ammonia.
Alternatively, styrene is first polymerised by low temperature (15 to 20 degrees Celsius) cationic polymerisation in a mixture of 96% strength sulphuric acid (90 parts by weight) and phosphoric acid fortified with phosphorus pentoxide (10 parts by weight). A small amount of surfactant (dipropylene glycol diphosphoric acid) is included. After polymerisation has proceeded sufficiently, the temperature is raised to effect sulphonation as described above. The reactor product is neutralised with ammonia.